Nucleic Acids and Nucleotides


  

Contents

  

    Nucleic Acid Bases
  

    Hydrogen-Bonding Sites in Nucleic Acid Bases
  

    Hydrogen-Bonded Nucleic Acid Base Pairs
  

    Sugars present in DNA and RNA


  


  Nucleic Acid Bases


DNA and RNA contain bases derived from pyrimidine (a nitrogen-base containing
a single aromatic ring) and purine (a nitrogen-base containing two fused
aromatic rings).  The structures of both the parent bases and the
derivatives commonly found in nucleic acids are shown below.

  
  

  
  



  


  Hydrogen-Bonding Sites in Nucleic Acid
  Bases


One of the most important structural features of polynucleotides is the fact
that they are able to participate in H-bonding.  The diagram below indicates
the H-bond donor and H-bond acceptor sites for the common nucleic acid bases.


  
  

  



  

    (A)
  

    H-bond acceptor. Requires a lone pair of electrons on an electronegative
    atom, such as F, O, or N.
  

    (D)
  

    H-bond donor. Requires a H atom covalently bonded to an electronegative atom.
    For example, H-F or H-O. While there is currently debate in
    the scientific literature about whether H-N bonds can form "true"
    hydrogen bonds, these type of interactions are very important in DNA and
    RNA and will be considered H-bonds in this course.


  


  Hydrogen-Bonded Nucleic Acid Base
  Pairs


Steric/spatial requirements in DNA require that a single pyrimidine interacts
with a single purine base. Two pyrimidine bases are too small, and cannot
get close enough together to form H-bonds. Conversely, two purine bases are
too large to fit into the gap separating the two strands of DNA.



The above diagram illustrates the Hydrogen-bonding pairs found in DNA. Note
that the A-T pair has only two H-bonds because of the adenine molecule. (The
lone pair of electrons on the oxygen atom in thymine molecule could act to
form a third H-bond, but adenine does not have the required donor). The
arrangement of donors and acceptors in thymine and guanine precludes formation
of H-bonding pairs between these two compounds.

  


  Sugars present in DNA and RNA




Ribose and deoxyribose are the sugars found in RNA and DNA, respectively.
The only difference in structure between these two molecules occurs at C2.



Shown above is a nucleotide, which consists of a base, a sugar,
and a phosphate group, with a second phosphate group attached to
C3. Shown below is a poly-nucleotide, linked by a phosphodiester
bond.



  


Last modified February 25, 1997

Kent State University - Stark Campus

Department of Chemistry

This Page Written and Maintained by Dr. Clarke Earley